add WithParseOption() to create jwtauth with custom parse options

[msvechla]

Hello,

This PR adds the new function WithParseOption(), which allows specifying custom parse options for JWTAuth. This enables specifying custom options such as jwt.WithKeySetProvider, which allows us to use JWKS and more sophisticated validation logic.

See also related #59, #40, #27. We should be able to resolve most of the mentioned issues here with these custom options.

Thanks a lot, let me know if you have any further feedback!

[msvechla]

We could also consider allowing to specify multiple parse-options

[msvechla]

@pkieltyka any thoughts on this?